Color images are defined by many pixels. Pixels are the building blocks of an image and are often defined by a three component description comprised of red, green and blue components commonly referred to as RGB. A fourth component called alpha is often added resulting in an RGBA description of pixels. Alpha refers to the opacity (or transparency) of a pixel. The standard alpha component is an eight bit value providing 256 levels of opacity from 0–255, where 0 is fully transparent and 255 is full opacity amounting to non-transparency. Alpha is popularly used in compositing images for feature films and other video presentations.
Compositing images is the combining of more than one image to create a single new image. Compositing of images can be performed with a single image frame, such as photograph or a sequence of images, such as film or movies. For example an image of a fish could be combined with the image of a grassy field so that the fish is swimming in the grass. Alpha is used to relate the fish image to the grass. For example, if the fish is placed on the grass and the grass is completely opaque (alpha=255), no grass will be seen through the fish. If the fish has an alpha of 0, the grass image will seep through the fish so that the fish cannot be seen. Alpha values in between produce differing degrees of a grassy fish. A fish swimming through the grass could be created by compositing a single image frame of a grassy field and a sequence of images created by filming a fish swimming in the ocean. Alternatively, two (or more) sequences of images can be composited, such as a sequence of images of a wind blown grassy field and a sequence of images of a school of fish swimming through the ocean. In addition, the school of fish and/or the grassy field can also have been created in an animation program. Moreover, the ocean area surrounding filmed fish could be obliterated by giving the surroundings (that is, everything but the fish) an opacity of 0, setting the surroundings to have no opacity.
Compositing also includes the techniques of enhancing a sequence of images captured on film by adding special effects, computer generated images, still images, animation and other filmed images to a motion picture. Compositing is used to enhance a sequence of film thus creating a new sequence of images. Special effects, computer generated images, still images, animation and other filmed images are combined with film by compositing to create a new sequence of images.
One popular use for compositing includes the use of blue screen image sequences. In what is known as “blue screening,” an actor is filmed in front of a blue (or green) screen. The alpha values of the background in such an image can be automatically set to transparent, having an opacity of 0. The blue screened sequence of images is then composited with other sequences of images. For example, a street vendor selling watches shot against a blue screen could be composited with a city street scene with English language signs behind him to create one sequence of image, while the same vendor sequence could be composited with Spanish language signs at one of Spain's costa del sol beach resorts to create a second sequence of images. More striking effects such as compositing sequences of images to show an actor riding a prehistoric dinosaur can be achieved in a similar manner.